An electronic circuit consumes power in various ways. For example, power is consumed when input signals to elements of the circuit change state. The power consumption occurs as the result of the charging and discharging of parasitic capacitances associated with the inputs to the circuit elements and with electrical conductors that provide the input signals to circuit elements. Power dissipation does not occur in the parasitic capacitance associated with a signal, but in the output resistance of the driver circuit which is the source of the signal.
A large portion of the power consumption in a modern integrated circuit ("IC") occurs in the signal drivers which drive the output pins of the IC and thus also the inputs to any other circuits or ICs connected to the output pins, typically by way of electrically conductive tracks formed on a printed wiring board ("PWB"). The parasitic capacitance associated with (i.e., driven by) each output signal driver is typically much higher than that associated with the internal signals within the IC. Consequently, the power dissipation is high.
A further problem associated with the output drivers of an IC results from the fact that the associated wiring, both within the IC package and on the PWB, is relatively long and therefore has high inductance. This inductance commonly causes undesirable signal characteristics such as "ringing" and "overshoot".
Referring to the drawings, FIG. 1a illustrates a conventional CMOS inverting output driver 10 formed as part of an IC. Output driver 10 generates an inverted driver output voltage signal V.sub.DO in response to a driver input voltage signal V.sub.I. Driver 10 is connected through electrical conductor 12 of a PWB to load circuitry 14. Specifically, electrical conductor 12 converts driver output voltage V.sub.DO) into a conductor output voltage signal V.sub.BO that drives a group of digital CMOS ICs 16 in load 14.
Driver circuit 10 is formed with N-channel insulated-gate field-effect transistor ("FET") QA and P-channel insulated-gate FET QB whose gate electrodes receive driver input voltage V.sub.I. The sources of FET QA and QB are respectively connected to a source of a low supply voltage V.sub.SS, typically ground reference (0 volt), and a source of a high supply voltage V.sub.DD. The QA and QB drains are connected together to provide driver output voltage V.sub.DO.
N-channel FET QA is turned on by raising input voltage V.sub.I to a suitably high level. On the other hand, FET QB is turned on by reducing voltage V.sub.I to a suitably low level. Accordingly, only one of FETs QA and QB is conductive during steady-state operation. If input voltage V.sub.I is high, FET QA is turned on to pull output voltage V.sub.DO to a low value close to V.sub.SS. Conversely, output voltage V.sub.DO is at a high value close to V.sub.DD when input voltage V.sub.I is low and causes FET QB to be turned on.
The "on" resistance of each of FETs QA and QB is normally quite low. Consequently, output voltage V.sub.DO makes a rapid transition from V.sub.SS to V.sub.DD when input voltage V.sub.I makes a high-to-low transition. Likewise, output voltage V.sub.DD makes a rapid transition from V.sub.DD to V.sub.SS when input voltage makes a low-to-high transition. During a transition, there is typically a brief period when both of FETs QA and QB are conductive.
PWB electrical conductor 12, commonly referred to as an interconnect, consists of a length of copper track and a ground plane at the V.sub.SS potential. The steps shown in the line passing through conductor 12 in FIG. 1a qualitatively represent the changes in direction that conductor 12 makes on the PWB. The ground plane is represented by the block in slanted shading. CMOS ICs 16 in load 14 are also variously connected to the V.sub.SS supply.
FIG. 1a does not explicitly show the various parasitic circuit elements which typically exist in any such circuit arrangement. For example, PWB conductor 12 is typically inductive and is also coupled to the nearby ground plane by parasitic capacitance. When driver 10 is formed as part of an IC within an IC package, the conductors internal to the package introduce further parasitic inductance and capacitance. Likewise, when ICs 16 are contained within IC packages, ICs 16 introduce further parasitic inductance and capacitance.
FIG. 1b shows a simplified electrical model of the circuitry in FIG. 1a. Inverting driver 10 is modeled by a switch SW in series with an output resistor R.sub.ON. Switch SW is controlled by input voltage V.sub.I. Resistor R.sub.ON represents the source-drain resistance of each FET QA or QB when it is turned on. PWB conductor 12 is modeled by parasitic inductance LB with distributed parasitic capacitance CB. Inductance LB and capacitance CB also model the parasitic inductance and capacitance of conductors internal to IC packages when driver 10 is formed as part of an IC contained in an IC package and/or when ICs 16 are contained within IC packages. Load 14 is modeled by parasitic capacitance CL representing the combined capacitances of the inputs to ICs 16 in load 14.
Consider a typical case in which conductor capacitance CB is much less than load capacitance CL. When input voltage V.sub.I causes switch SW of FIG. 1b to change state, output voltages V.sub.DO and V.sub.BO change in the manner generally illustrated by the waveforms of FIGS. 2a-2c. Since capacitance CB is much less than capacitance CL, the combination R.sub.ON, CL, CB, and LB approximates a series LC resonant circuit which can be (a) underdamped, (b) critically damped, or (c) overdamped according to the value of on resistance R.sub.ON compared to the reactance of inductance LB.
The waveforms shown in FIG. 2a correspond to the underdamped case in which resistance R.sub.ON is very low. FIG. 2a illustrates how output voltages V.sub.DO and V.sub.BO generally vary when input voltage V.sub.I makes a high-to-low transition. Driver output voltage V.sub.DO rises quickly from V.sub.SS to V.sub.DD. The effect of conductor inductance LB is to limit the initial flow of current from driver 10. Consequently, conductor output voltage V.sub.BO changes slowly at first. However, once current has started flowing through inductance LB, the current continues to flow even when conductor output voltage V.sub.BO has reached V.sub.DD. This leads to overshoot in conductor output voltage V.sub.BO and consequent ringing.
FIG. 2b shows waveforms generally representative of the critically damped case in which resistance R.sub.ON is moderately (but not very) low. The behavior is similar to the underdamped case except that just enough energy is dissipated in resistance R.sub.ON so that there is little overshoot in conductor output voltage V.sub.BO, substantially no ringing, and the transition speed for both of output voltages V.sub.BO and V.sub.DO is moderately high.
FIG. 2c shows waveforms corresponding to the overdamped case where resistance R.sub.ON is relatively high. Here, the transition speed for both of output voltages V.sub.DO and V.sub.BO is low. In particular, conductor output voltage V.sub.BO takes a comparatively long time to reach the desired V.sub.DD level.
Both overshoot and low transition speed are generally undesirable signal characteristics. Consequently, the waveforms shown in FIG. 2b often represent the best case for conventional driver 10. For any of the circuit behaviors depicted in FIGS. 2a-2c, the ringing frequency, the transition speed, and the degree of overshoot depend on the values of elements R.sub.ON, LB, CB, and CL. While the value of on resistance R.sub.ON can be controlled during driver design, the values of elements LB, CB, and CL vary from one PWB conductor to another and from one load to another, making it very difficult to achieve the often desired waveforms of FIG. 2b in the typical case where driver design is completed without knowledge of the specific load characteristics.
The energy drawn from the V.sub.DD power supply approximately equals CV.sup.2, where V is the potential difference V.sub.DD -V.sub.SS, and C is the sum of capacitances CB and CL. Approximately half the supplied energy--i.e., 1/2CV.sup.2 --is stored in load capacitance CL. The remainder of the supplied energy--i.e., also approximately 1/2CV.sup.2 --is dissipated in on resistance R.sub.ON.
Similar waveforms occur when input voltage V.sub.I makes a low-to-high transition, causing output voltages V.sub.DO and V.sub.BO to make high-to-low transitions. In this case, no energy is drawn from the V.sub.DD supply. However, the energy stored in load capacitance CL is substantially dissipated. Accordingly, the average power drawn from the V.sub.DD supply when input voltage V.sub.I makes transitions at an input frequency f.sub.I is f.sub.I CV.sup.2. The majority of the power is dissipated in driver output resistance R.sub.ON. The load itself typically contains relatively insignificant loss mechanisms.
In the case where on resistance R.sub.ON is very low, only a small amount of power dissipation occurs during the initial output transition. Unfortunately, the ringing persists for a relatively long time. Power is gradually dissipated in driver output resistance RON, causing the amplitude of the ringing to decay gradually. This is quite wasteful of power. It is desirable to reduce the power consumption in a driver circuit without sacrificing other desirable performance characteristics and without reducing the capability to accept loads of greatly varying types.